Nitrate ion sensitive electrode

ABSTRACT

An electrode for measuring the concentration of nitrate ions in an aqueous solution wherein the sensing portion is an organic phase containing a high molecular weight quaternary ammonium salt dissolved in a high molecular weight nitrophenyl-type solvent.

United States Patent 1 June 20, 1972 3,429,785 2/1969 Ross ..204/] T3,448,032 6/1969 Settzo et a1. .....204/195 3,483,112 12/1969 Ross..204/195 Primary Examiner-T. Tung AttorneyClarence R. Patty, Jr.,Clinton S. James. Jr. and James A. Giblin [57] ABSTRACT An electrode formeasuring the concentration of nitrate ions in an aqueous solutionwherein the sensing portion is an organic phase containing a highmolecular weight quatemary ammonium salt dissolved in a high molecularweight nitrophenyl-type solvent.

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INVENTOR.

Warren M. Wise gm AM ATTORNEY PATmTEnJunzolmz I 3,571,413

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6 q) '1.- um 1 1 l l N O o o o 0 It 2' w v I b l l o 4 s 3W3 INVENTOR.Warren M. Wise I BY AT TO RNEY Electrodes for determining ionconcentrations in aqueous solutions are well known. Electrodes have beendesigned to measure the concentration of such cations as H*, Na, K, Ag*,Cu", and Ca as well as the concentration of such anions as Cl, Br, I,and F. For any of the above measurements, two electrodes are needed. Oneelectrode is called the sensor and the other is called the referenceelectrode. In use, the two electrodes are commonly connected to a highimpedance potentiometer and then immersed in a test solution. Dependingon the construction of the sensor electrode, and the extent of ionicactivity in the test solution, an electrochemical cell may develop. Thepotential of this cell can be read on the potentiometer. Since ionicactivity is a measure of ion concentration, the potential reading can betranslated into a measure of ion concentration in a given test solution.

The choice of sensor electrode will depend on the type of ion whoseconcentration is to be determined. In the case of cation concentrationdeterminations, the sensing electrode should be sensitive to cationicactivity; contrariwise, where anionic concentrations are to bedetermined. For a sensor electrode to be usefully sensitive to aparticular ion, the electrode must be of such a nature that it sensesthe activity of that particular ion in preference to the activities ofother ions that may be present in the test solution.

The preference of a sensing electrode for certain ions is referred to asthe selectivity of the sensor electrode for certain ions over otherions. This selectivity is governed by the tendency of the sensitiveportion of the sensor electrode to sense given test ions over other ionsat the same concentration. Thus, if the sensitive portion of the sensorelectrode is of such composition as to more readily sense the test ionactivity, the EMF noted on a potentiometer will be mainly attributableto the test ion activity. This, in turn, provides an indication of testion concentration.

The dependence of potential change on the activity of the specific ionto be tested follows the well-known Nemst equation for a givenconcentration range and is related to the physical-chemicalcharacteristics of the electrodes. The Nernst equation shows:

E,,, E, RT/nF In a, E, 0.059/n log a, 25 c.

where E,,, is the electrode potential, E is a constant, n is the 7 ioncharge (I), and a, is the activity of the specfic ion in solution.

It is apparent from the equation that a change in activity (a,)equivalent to one order of magnitude causes 59 mV potential change whenthe ion is univalent and about 30 and mV, respectively, when the ion isbivalent or trivalent. Thus, the reliability of a given sensor electrodeat given concentrations of a known univalent anion such as N0 (where n,the ion charge, is l can be shown if there is a potential difference ofabout minus 59 mV for each logarithmic increase in concentration.

PRIOR ART In practice, the sensitive portion of the sensor electrode isdesigned in such a way as to limit the manner in which contact is madewith the aqueous test solution. Thus, the useful life span of aparticular electrode is in no small part determined by how well thesensor material can be prevented from substantially leaving the sensorelectrode and undesirably mixing with the aqueous test solution. At thesame time, however, care must be taken to assure that the sensor can bebrought in contact with an aqueous test solution to provide an interfacefor sensor-test solution interaction.

There are many well-known methods for accomplishing the above goals. Forexample, various porous membranes have long been used to keep the sensorphase mechanically separated from the test phase while allowing forelectrochemical interaction between the two. Among the more recentmethods disclosed is that by Ross (US. Pat. No.

3,429,785) wherein various organic ion exchangers in organic solventsare demonstrated as being useful for measuring ion concentrations inaqueous solutions. Being substantially hydrophobic, such organic ionexchange electrodes greatly limit the amount of sensor-solvent leakageinto the test solution, thereby prolonging sensor electrode life.Another recent disclosure by Settzo and Wise (US. Pat. No. 3,448,032)shows that a liquid organic ion exchange electrode can be made with anorganophilic-hydrophobic membrane separating the sensor and testsolution.

The above disclosures make clear the importance of maintaining theseparation of the sensor and test solutions or at least delaying theirrate of mixing.

Hand-in-hand with the problem of maintaining a separation between thesensor and test solution is the problem of providing a sensor electrodethat will give Nemstian responses for a particular ion over other ionsat the same concentration. Since sensor electrode selectivity forcertain ions over others determines the utility of sensor electrodes, itis important that the electrode preference for the ion to be measured beas high as possible over other ions that might be encountered in thetest solution.

Although electrodes sensitive to anions such as N0 have been disclosed(e.g. Ross, US. Pat. No. 3,483,] 12), the selectivity for N0 over otheranions, especially Cl ions, presents a limitation in situations where itis desirable to measure N0 concentration in the presence of varyingamounts of Cl ions, Further, in spite of the hydrophobicity of suchknown sensors, there still remains an undesirable amount ofsensor-to-test solution leakage. This leakage, as noted, is a limitationon the useful life span of a sensor electrode. Thus, the followingdiscovery was surprising in that it minimizes the above two limitationssimultaneously.

SUMMARY OF THE INVENTION I have now made an electrode which isparticularly sensitive to nitrate ions in the presence of chloride andother anions. With this electrode, the selectivity for N0 in thepresence of chloride ions in a 10 molar solution is about 150 to 1.Thus, this electrode is capable of measuring nitrate ions in thepresence moderate amounts of chloride ions without the necessity formaking background corrections. Further, in the optimum combination ofsensor and organic solvent, the sensing phase has a relatively highviscosity (e.g., greater than about centipoises) which, in conjunctionwith its hydrophobicity promotes a longer useful electrode life.

Specifically, l have found the above qualities can be achieved where theelectrode for measuring nitrate ion concentrations in aqueous solutionscomprises an organic sensor phase containing an ion-exchange materialcomprising a high molecular weight quaternary ammonium salt dissolved ina high molecular weight nitrophenyl-type compound. Preferably, themolecular weight of the quaternary ammonium salt should be greater thanabout 400, and the molecular weight of the solvent should be greaterthan about 200. The sensor and solvent may be contained by anyconventional sensor electrode body as long as provision is made for aninterface between the sensor phase and the aqueous solution. An internalreference electrode in electrical contact with the sensor must, ofcourse, also be provided. This internal electrode may be of anyconventional type such as an Ag/AgCl electrode immersed in a chloridesolution which, in turn, is in electrical contact with the sensor phase.To complete the circuit in use, a high impedance potentiometer and aconventional reference electrode such as a saturated calomel referenceelectrode may be used. Thus, the electrochemical cell can be representedas follows:

Ag/AgCl, Chloride Solution l Sensor Phase Test Solution Sat. CalomelSPECIFIC EMBODIMENTS The invention may be more clearly understood fromthe following description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a diagrammatic illustration showing a testing circult of whichthe present electrode is a part.

FIG. 2 is a partial cross-sectional view of a representative electrodewhich can employ the principles of the present invention.

FIG. 3 is a chart showing the relative selectivities for various anionswhen the high molecular weight quaternary ammonium salt istridodecylhexadecylammonium nitrate and the high molecular weightnitrophenyl-type solvent is n-octyl-onitrophenyl ether.

FIG. 4 shows the efiect of pH change on the above electrodes nitrateresponse.

FIG. 5 shows the effect of pH change on the above electrode s chlorideresponse.

Returning to FIG. 1, it can be seen that the electrode of the presentinvention is in circuit with a potentiometer 2, a reference electrode 4,and a test solution 6. Depending on the ionic activity of test solution6, an effect on potential will be observed at potentiometer 2.

Referring now to the illustration of FIG. 2, the ion exchange electrode10 embodying the principles of the present invention is comprised of anelectrically insulating container such as an outer glass tube 12 havingan opening at each end thereof. Only the sensing end of the glass tube12 is shown in cross section. The sensing end of the tube 12 is tightlycapped with a substantially chemically inert porous glass membrane 14which is attached to the glass tube 12 by a suitable means such assolder glass 16 or directly by a glass-to-glass seal. The interiorportion of the glass tube 12 contains the sensor phase material 18 ofthe present invention, i.e., a high molecular weight quaternary ammoniumsalt dissolved in a viscous, high molecular weight nitrophenyl-typecompound. When assembled, and in actual use, the sensor phase is incontact with and fills the pores of the membrane 14. The high viscosityof the sensor and its solvent and their hydrophobicity, however, greatlyminimize any sensor-solvent leakage into the aqueous testing phase.Shown within the sensor phase is an internal reference electrode 23,which is preferably of the Ag/AgCl type. This internal referenceelectrode 23 consists of an inner glass tube 24 which can be attacheddirectly to the porous glass membrane 14 by means of solder glass 17, aplatinum wire 22 with a Ag/AgCl coating 30, and a salt bridge 34consistingof saturated chloride solution.

The end of the outer glass tube 12 is suitably capped by a lid 36 whichacts both as a closure and a support for an electrically conductive lead38 in electrical contact with the platinum wire 22.

In this particular embodiment, the inner salt solution 34 is separatedfrom the ion exchange liquid 18 by means of the porous membrane 14. Theion-exchange liquid 18 flows downward and laterally into the pores ofthe membrane 14 and thus comes in electrical contact with both the saltbridge solution 34 and the test solution. The high viscosity of thesensor and solvent 18 severely limits any leakage into either theaqueous phase or the salt solution 34.

It is thought that the sensor electrode senses ion concentrations in thefollowing manner: when the sensor and standard reference electrodes areimmersed in the test solution, a circuit is completed. The Ag/AgCl insaturated chloride solution gives rise to a potential. Since thechloride solution is in electrical contact with the aqueous testsolution by means of the sensor material, the above potential isaffected by the test solution ion activity. This activity is sensed atthe interface between the sensor material and the aqueous test solution.Since the sensed activity of the test solution is related to itsconcentration, the effect of the test ions on the potential reflects thetest ion concentration. In anion concentration measurements, a greaterconcentration of anions will affect the. potential by a negative factor.Thus, under the Nemst equation, an anion such as NO;, will bring aboutan 59 mV decrease in potential for each logarithmic increase inconcentration.

As noted earlier, various anion sensors have been disclosed. However,due to the relatively short life span of electrodes made from such knownsensor materials, attention was directed toward finding sensor materialsthat would simultaneously provide good N0 selectivity and also have along life.

Many classes of sensor solvents were investigated; however. it was foundthat high molecular weight nitrophenyl compounds were most suitable whenan anion sensitive material such as a quaternary ammonium salt was usedas the ion sensor. After certain alkyl-nitrophenyl ether compounds wereincluded in this investigation, it was discovered that these solventspromoted NO; over Cl selectivity. Therefore, the following work was doneto determine the optimum combination of solute and solvent required forthe preparation of a sensor that would display the best characteristicsfor a NO; electrode. It should be pointed out that sensors and solventsof greater molecular weight than those disclosed immediately below canbe used. However, particular attention was given to high molecularweight compounds that can be made from commercially availableingredients.

All electrochemical measurements discussed below were made in unstirredsolutions with the nitrate electrodes so made and a saturatedcalomel-electrode connected to a Corning Model l2 Research pH meter. Allreagents were prepared using analytical reagent grade compounds andtwice distilled water.

SOLVENTS FOR SENSOR MATERIAL After it was discovered that certainaIkyl-nitrophenyl ether compounds, when used as solvents, promoted N0over Cl selectivity, attention was directed toward discovering the idealsolvent from this class. It was noted that, in general, the orthoalkyl-nitrophenyl ethers have lower melting temperatures than theircorresponding meta and para position isomers. Consequently, if themolecular weights of most of the meta and para alkyl-nitrophenyl ethersare sufficiently high for them to be desirably insoluble in water, andhave high boiling temperatures, the compounds are usually solids at roomtemperature. Therefore, since the investigation was concerned withliquid specific ion sensors, most of the attention was given to theortho derivatives.

The compounds shown in table 1 were prepared by wellknown methods.

TABLE I Some Ethers Prepared for Investigation As Solvents for LiquidAnion Sensors p-chlorobenzyl-onitrophenyl ether (solid at room temp.)

Usually, the compounds that were liquids at room temperature possessedhigh viscosities and densities near I ION EXCHANGE MATERIAL about 150times that of the N Thus. this greater selectivity for N0 over Cl ionsrequires essentially no background When certain uaternar ammonium saltswere dissolved in q y corrections be made when Cl ions are present inappreciable the above ethers, anion sensors were obtained. Some saltsinquantities. volved in thisinvestigation that produced useful sensorswere: 5 A f rth check on the usefulness f the above electrode"ioctylpropylammohium nitrate was made by checking the responses to bothN0 and Cl tetraheptylammonium Chloride ions when the pH of the testsolutions were varied from about tridodefiyl'P'chlorohehzylammoniumChloride 3 to about 9. As can be seen in FIG. 3, variations in pH fromtrlodecylbehzylammohium carbonate about 3 to about 9 had substantiallyno effect on the sensor trioctyl-p-nitrobenzylammonium chloride 10responses to N0 concentrations of 10 to 10 M.carbethoxymethyltridodecylammonium nitrate, and Likewise, in FIG. 4, theresponses of the same concentrations tridodecylhexadecylammonium nitrateof Cl ions, to which the electrode was much less sensitive. When thelast salt mentioned in the above list was present in nwere similarlyunaffected. octyl-o-nitrophenyl ether, there resulted the best N0 sensorThus, it can be seen that the electrode of this invention is foundduring the entire investigation. Optimum performance particularly usefulis measuring the concentration of N0 resulted when the concentration ofthe salt was adjusted to 2 ions in test solutions which include a widerange of Cl conpercent (W/V). centrations, and/or a wide variation inhydrogen ion concen- The electrolyte for the internal Ag/AgCl electrodeshould tration. preferably be saturated NaCl solution. If KCl is used,the elec- I Sh l be noted that the principles of this invention aretrode will display greater than Nemstian responses for all not limitedto the ideal sensorsolvent combination disclosed anions at the lowconcentrations, because alkylnitrophenyl abovey Ofthe high molecularWeight quaternary ammohi' ethers alone have weak K responses; but whenNaCl is used um Salts disclOsed Provided useful Sensors when dissolvedin a this behavior is eliminated. Also, the solution should be Suitablehigh molecular Weight p y p COmPOUhd in slightly acidic to prevent thereaction of the Ag with the Such Proportions to Produce a Sensingsolution of relatively quaternary ammonium salt at the interface of theinternal high Viscosityelectrolyte and the organic sensor. Anaccumulation of the Further the serfsorsoivem cfmblhahohi disclosed.above products of this reaction is visible as a dark brown residue thatcan be employed a wlde vanety of electrode bodles the has deleteriouseffects on the electrodes characteristics. only requlreme'l} bemg ameans for provlduig Interface for sensor-test solution contact and ameans for internal reference PREPARATION OF SENSOR lN SOLVENT 3electrode contact with the sensor. The sensor phase of the I aboveelectrode need only be the sensor phase of a cell such as Beforedetermining that tridodecylhexadeylammonium the followingelectrochemical cell: nitrate dissolved in n-octyl-o-mtrophenyl etherwas an ideal sensor, the combination of sensor and solvent was preparedin Ag/AgCl, Chloride Solutionl Sensor Phasel Test Solution i thefollowing manner: 5 Sat. Calomel 6 grams of tridodecylhexadecylammoniumcarbonate were 3 added to 70 ml. of n-octyl-o-nitrophenyl ether, and themixture was stirred several hours without heating until the solid wascompletely dissolved. The solution was transferred to a 250 ml.separatory funnel and washed three times with 50 ml. 40 portions of l MNaNO then twice with distilled water. The organic phase was filteredthrough two stages of glass wool to remove the water. The filter waswashed several times with noctyl-o-nitrophenyl ether, and the combinedfiltrate and washings were diluted to 300 ml. with moren-octyl-onitrophenyl ether.

The above vertical lines represent material boundaries.

Thus, those skilled in the art will recognize other possible variations.Accordingly, it is intended that this invention be limited in scope onlyby the appended claims.

I claim:

1. An electrode for measuring the concentration of nitrate ions in anaqueous solution and comprising in combination:

a. an organic nitrate ion sensor phase comprising a quaternary ammoniumsalt selected from the group consisting of trioctylpropylammoniumnitrate tetraheptylammonium chloride,

ELECTRODE ASSEMBLY tridodecyl-p-chlorobenzylammonium chloride,

tridodecylbenzylammonium carbonate,

Once the above sensor-solvent preparation was made, thetrioctyl-p-nitrobenzylammonium chloride, liquid anion-exchangeelectrodes were assembled and used in carbethoxymethyltridodecylammoniumnitrate, and a conventional manner. However, to obtain optimumpertridodecylhexadecylammonium nitrate; fonnance of the N0 electrode, itwas preferably to use, as its b. a substantially viscous solvent for thesalt of (a) and cominternal electrolyte for the Ag/AgCl electrode, asaturated prising an alkylnitrophenyl ether having a molecular solutionof NaCl containing 0.008 M HNO to bring about the Weight greater thanabout 200, said solvent being substandesired slight acidity noted above.tially organophilic and hydrophobic;

Once the above electrode was assembled and ready for use, c. means forcontaining the organic sensor phase of a in known concentration ofperchlorate, iodide, nitrate, bromide, the solvent of 50 as t0 providean interface r i n and chloride were found to give Nemstian responseswhen the e c ange ontact between the materials so contained and sensorcomprised tridodecylhexadecylammonium nitrate disa(18011S Solution; and,solved in n-octyl-o-nitrophenyl ether. That is, for each d. an internalreference electrode element in electrical conlogarithmic change inconcentration, each of the anions tact with the materials Cohmihed y(Shown in chart f FIG 3 brought about a 59 y change i 2. The electrodeof claim 1, wherein the solvent of (b) is npotential. As shown by thechart of FIG. 3, the electrode was y p y etherparticularly sensitive toClO, and l anions and, as noted, electrode i wherem the q f- -U: theseions should be absent or extremely minimal when NO; momum of (a) 15thdodecylhexadecylammomum "mate and the solvent of (b) isn-octyl-o-nitrophenyl ether.

4. The electrode of claim 3, wherein the concentration of the salt of(a) in the solvent of (b) is about 2 percent by weight/volume.

concentrations are being measured. However, as can be seen by the samechart, the selectivity for N0 ions over Cl ions was about 150:1 atconcentrations of 10 m. This means that in order for the Cl ions toestablish the same potential as the N0 ions, the concentration of the Clwould have to be

2. The electrode of claim 1, wherein the solvent of (b) is n-octyl-o-nitrophenyl ether.
 3. The electrode of claim 1, wherEin the quaternary ammonium salt of (a) is tridodecylhexadecylammonium nitrate and the solvent of (b) is n-octyl-o-nitrophenyl ether.
 4. The electrode of claim 3, wherein the concentration of the salt of (a) in the solvent of (b) is about 2 percent by weight/volume. 